This invention relates to an improvement of magnetic dispersion. Concretely, it relates to an improvement in the hue of magnetic dispersion.
The term "magnetic dispersion" means a liquid in which magnetic fine particles having a particle size of about 50-200 .ANG. are dispersed in a dispersion medium by the aid of a surfactant. Such a magnetic dispersion has a characteristic property that it is stable for a long period of time, without precipitation nor agglomeration.
For the magnetic fine particle, fine particles of ferrite compounds such as magnetite, manganese ferrite, nickel ferrite, cobalt ferrite, iron zinc ferrite, manganese zinc ferrite, nickel zinc ferrite, barium ferrite and the like are in use. As a surfactant, carboxylic acids such as oleic acid, linoleic acid and the like, cationic surfactants, nonionic surfactants or the like are used either alone or in combination. As the dispersion medium, hydrocarbon compounds such as kerosene, toluene and the like, ester compounds such as ester oil and the like, ether compounds, fluorinated hydrocarbon compounds and the like are used either alone or in combination.
As a use for the magnetic dispersion, there can be mentioned a wide variety of uses including the uses in the mechanical field such as sealant, lubricant and the like; the uses in the field of separation such as an agent for gravity concentration, an agent for oil-water separation and the like; the uses in the field of printing and recording such as developer for magnetic or electrostatic latent image, ink for ink jet and the like; the uses in the field of toys; and so on.
Though the magnetic fluid of this invention can be employed in all the above-mentioned use fields of magnetic dispersion, it exhibits a particular usefulness in the field of printing and recording. Accordingly, in order to promote the understanding of the magnetic fluid of this invention, its application to the magnetic fluid recording process using a magnetic dispersion as an ink will be illustrated below. It is needless to say that the illustration lays down no unnecessary limitation on this invention.
Magnetic fluid recording process is a process for making records according to which the above-mentioned magnetic dispersion is used as an ink and a record is made by letting the ink fly, migrate or deflect by the action of pressure, magnetic force, Coulomb force or the like.
In this field, it has hitherto been proposed to use magnetic dispersion either as it is or after diluting it with a dispersion medium. The above-mentioned magnetic dispersion has had a drawback that, though it assumes a black or black-brown color usually, its hue changes when it is formed into a thin layer for making a record on a recording member or when it permeates into the recording member to form a record. The reason for this color change is believed to be as follows. The magnetic fine particles in the record image formed on a recording member are very small in number and the particle size of the magnetic fine particle used in magnetic dispersion is generally unsuitable for absorbing light having long wavelength. Thus, the hue of the record image has an increased redness as compared with the magnetic dispersion before recording and looks black-brown or light brown. Further, the hitherto known magnetic dispersion has had an additional drawback that, when it is used as an ink, the hue of the record made therefrom is limited to the color of the magnetic fine particle per se.
Accordingly, in order to form a record having a color other than a brownish color by using the magnetic dispersion as an ink, addition of a coloring material has been necessary.
Magnetic dispersion are roughly classified into magnetic dispersions using an organic dispersion medium which have a relation to this invention (hereinafter, they are referred to as "organic magnetic dispersions") and magnetic dispersions using water as dispersion medium (hereinafter, they are referred to as "aqueous magnetic dispersions").
They are not only different from each other in dispersion medium, but they are greatly different also in the construction of magnetic fine particle and surfactant. Thus, in the organic magnetic dispersion, the hydrophilic group of surfactant is adsorbed on magnetic fine particle so that the hydrophobic group of the surfactant is directed towards the dispersion medium. That is to say, surfactant forms a monolayer surrounding the magnetic fine particle. On the other hand, in aqueous magnetic dispersion, surfactant forms a double layer to surround a magnetic fine particle. That is to say, the hydrophobic group of the surfactant of the first layer is adsorbed on the magnetic fine particle, and the hydrophobic group of the surfactant of the first layer confronts the hydrophobic group of the surfactant of the second layer, so that the hydrophilic group of the surfactant of the second layer is directed towards the dispersion medium.
As compared with a micelle structure consisting only of one layer, this double layer structure is inferior in dispersion stability due to the weak adsorption force between the hydrophobic groups. Accordingly, if a physical, chemical or electrical action of electric field, strong flow, heat or the like is exercised thereupon, the magnetic particle in aqueous magnetic dispersion readily undergoes precipitation or agglomeration. Further, since the vapor pressure and viscosity of water are dependent only on temperature and humidity, the viscosity of aqueous magnetic dispersion is almost unchangeably fixed, only depending upon the temperature and the content of magnetic fine particle, so that the viscosity is quite difficult to control. Further, when it is used in an atmosphere having a low humidity, the water vaporizes rapidly so that the concentration of magnetic fine particle in the aqueous magnetic dispersion rises owing to the vaporization of the dispersion medium (water) (i.e. concentration) or precipitation of the magnetic fine particles can occur. Such concentration and precipitation of magnetic fine particles cause a change in the properties of the magnetic dispersion such as viscosity, electrical properties, magnetization and the like. Accordingly, if an aqueous magnetic dispersion is used as, for example, the ink for ink jet, clogging of the nozzle readily takes place. Further, in the general magnetic fluid recording process, the conditions of flying, migration or deflection of ink readily change, which has been an important problem in continuing the recording for a long period of time or using the ink (aqueous magnetic dispersion) after a longterm standing. Further, as compared with organic magnetic dispersion, an aqueous magnetic dispersion has a lower electric resistance, so that it is difficult to use in the magnetic fluid recording process wherein a magnetic dispersion is let fly only by the action of Coulomb force obtained by applying a voltage, because the applied voltage leaks and produces no Coulomb force. Further, an aqueous magnetic dispersion cannot be used as a lubricant because water is used as dispersion medium in it. Further, as compared with the case of organic magnetic dispersion, aqueous magnetic dispersion is inferior in dispersion stability and the vaporization speed of dispersion medium and the viscosity are more difficult to control, so that the use of the aqueous magnetic dispersion is quite limited among the above-mentioned use fields of magnetic dispersions. Mainly, the possibility of utilizing it as an ink for ink jet using deflection by magnetic field is being discussed.
Although aqueous magnetic dispersion has an advantage that, owing to the use of water as dispersion medium, its hue can be changed easily by mixing a number of coloring materials (particularly dyestuffs) thereinto, it has many drawbacks mentioned above which are a great obstacle to the use of aqueous magnetic dispersions.
On the other hand, the organic magnetic dispersion relating to this invention is excellent in the dispersion stability of magnetic fine particles, as has been mentioned above. Accordingly, it keeps stable against the physical, chemical or electrical actions of electric field, strong flow, heat or the like. Further, with regard to vapor pressure of dispersion medium it still has an advantage that a dispersion medium having a vapor pressure suitable for the environment of use can easily be selected and used. That is, a dispersion medium having a larger number of carbon atoms can be used as a dispersion medium having a lower vapor pressure, and a dispersion medium having a smaller number of carbon atoms can be used as a dispersion medium having a higher vapor pressure. Further, by using a mixture of organic compounds different in carbon number as a dispersion medium, vapor pressure can be controlled more minutely and viscosity of magnetic dispersion can also be controlled so as to meet the purpose of use.
For these reasons, an organic magnetic dispersion has a merit that the problems due to vaporization of dispersion medium which caused concentration and precipitation of magnetic fine particles are much less than in the case of an aqueous magnetic dispersion. Accordingly, for example, the clogging of nozzle in ink jet takes place less easily than in the case of aqueous magnetic dispersion and the conditions of flying, migrating and deflecting of magnetic fluid less readily change than in the other case even in the general magnetic fluid recording process, so that a stable record can be obtained when used in a longterm continuous recording or in a recording after a longterm standing of ink. Further, since organic magnetic dispersion is generally higher than aqueous magnetic dispersion in electric resistance, it can be used in extensive recording processes including the magnetic fluid recording process according to which a voltage is applied and a magnetic dispersion is let fly only by the action of Coulomb force. Further, organic magnetic dispersion can be extensively used not only in the fluid of printing and recording but also in all the above-mentioned uses of magnetic dispersions, so that its industrial utilization is much higher than that of aqueous magnetic dispersion.
However, the hue of organic magnetic dispersion is quite difficult to change, which has been an important drawback of organic magnetic dispersion.
That is, the hitherto well known coloring materials cannot readily dissolve nor disperse into organic magnetic dispersions. Therefore, changing the hue of a magnetic dispersion has been possible hitherto only by sufficiently diluting the magnetic dispersion with a dispersion medium and then mixing coloring materials thereinto. On the other hand, the magnetization of magnetic dispersion is approximately proportional to the content of magnetic fine particle in magnetic dispersion. Accordingly, if the hue of a magnetic dispersion is changed by the prior art measure in the above-mentioned manner, namely by sufficiently diluting the magnetic dispersion with a dispersion medium and then mixing coloring materials thereinto, its magnetization becomes extremely low and, in some extreme cases, the response to magnetic field is hampered so that the magnetic characteristics of a magnetic dispersion can be lost substantially. In other words, there has been a drawback that an intention to change the color of magnetic dispersion results in damaging its magnetic characteristics, and inversely, an intention to maintain its magnetic characteristics results in impossibility of changing the color. Further, if pigment particles having a form of colloidal particle are merely mixed into an organic magnetic dispersion, said pigment particles gradually agglomerate and then precipitate so that a stable magnetic dispersion capable of retaining its hue for a long period of time cannot be obtained.
An object of this invention to provide a magnetic fluid containing organic compounds as the dispersion medium, excellent in magnetic characteristics, and having a rich and stable color.
The object of this invention can be achieved by using a magnetic fluid formed by mixing an organic magnetic dispersion with a colorant prepared by previously treating a coloring material with a solubilizing treating agent or a dispersibilizing treating agent.